The Diagram Below Shows the Marginal Cost of Pollution Abatement

The diagram below shows the marginal cost of pollution abatement for two firms,Firm X and Firm Y.
FIGURE 17-4
-Refer to Figure 17-4.Suppose Firms X and Y are each abating QR units of pollution.If Firm X then reduces pollution by one unit and Firm Y increases pollution by one unit,then

A) total pollution remains the same but the total cost of abatement falls.
B) total pollution remains the same but the total cost of abatement increases.
C) the total cost of abatement remains the same but pollution falls.
D) the total cost of abatement remains the same but pollution increases.
E) the total costs of abatement and the amount of pollution do not change.